Robert Spragg

Factors That Influence Electrical Resistivity Measurements in Cementitious Systems

The electrical resistivity of cement-based materials can be used in quality control or service life prediction as an indicator of the fluid transport properties of these materials. Although electrical tests have the advantage of being easy and rapid to perform, several key factors can influence the results: (a) specimen geometry, (b) specimen temperature, and (c) sample storage and conditioning. This paper addresses these issues and compares the measurements from several commercially available testing devices. First, the role of sample geometry is explained with the use of three common geometries: surface, uniaxial, and embedded electrodes. If the geometry is properly accounted for, measurements from different test geometries result in electrical resistivity values that are similar. Second, the role of sample temperature is discussed for both pore solution and uniaxial tests on cylinders. Third, the paper examines the importance of sample curing, storage, and conditioning. Sample storage and conditioning influence both the degree of hydration and the degree of saturation. The role of sample volume to solution volume is discussed, as this ratio may influence alkali leaching and pore solution conduction. This paper is intended to identify factors that influence the results of rapid electrical test measurements and to help identify areas of future research that are needed so that robust specifications and standard test methods can be developed. Standardization will enable electrical tests to provide rapid, accurate, repeatable measurements of concretes electrical properties.

Water Absorption and Electrical Conductivity for Internally Cured Mortars with a W/C between 0.30 and 0.45

Internal curing has emerged over the last decade as an approach to counteract the negative effects associated with self-desiccation in low water-to-cement ratio (w/c) mixtures. Specifically, much of the early research on internal curing focused on the reduction of autogenous shrinkage. Recent work has demonstrated, however, that internal curing can also be beneficial in reducing drying-shrinkage cracking, reducing the propensity for thermal cracking, reducing fluid absorption, and reducing ion diffusion in concrete. However, several aspects of internal curing still require closer examination. One of these aspects is the application of internal curing for mixtures with a wider range of water-to-cement ratios. This paper describes results from experiments that investigated the potential use of internal curing in mortar systems with w/c ratios of 0.30, 0.36, 0.42, and 0.45 that were cured under sealed conditions, in terms of water absorption and electrical conductivity. Test results show that internal curing...

Comparison of the Pore Volume in Concrete as Determined Using ASTM C642 and Vacuum Saturation

For the prediction of concrete performance, it is often beneficial to know the volume of pores in the concrete. ASTM C642-13 [Standard Test Method for Density, Absorption, and Voids in Hardened Concrete, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA] was originally introduced in 1969 as a “standard test method for density, absorption and voids in hardened concrete.” This technical note will compare the total pore volume (voids) obtained using ASTM C642-13 with that obtained using vacuum saturation for a typical bridge deck concrete. This technical note suggests that ASTM C642-13 may provide a consistent measure of the pores (voids) in the paste matrix of the concrete; however, it does not provide a measure of the total porosity that may be expected if all the pores in concrete are filled with water. Although an approach like ASTM C642-13 may be sufficient for some applications, the fact that it does not measure the total porosity should be considered by users of the test. This is important because this test method is being frequently applied to concrete durability and transport-related problems that require the total pore volume to be known.

Portland Cement Concrete Pavement Permeability Performance

The objective of this project was to evaluate the transport properties of concrete pavement in the state of Indiana using common testing procedures. Specifically this work evaluated the absorption of water, the absorption of deicing solutions, and electrical conductivity. A series of concrete paving mixtures were tested to provide a range of values that were typical for the state of Indiana. While similar mixture proportions were used for the mixtures in Indiana differences in the magnitude of water absorbed occurred. A series of mortars were tested to illustrate the effect of curing conditions, water to cement ratio, and paste volume. It was observed that a long duration of drying was needed to obtain equilibrium. Samples dried to a lower relative humidity showed a greater volume of water absorbed. It was observed that drying at 105C resulted in substantial anomalies in water absorption, and as such this method is not recommended. It was observed that when samples were tested using deicing solutions or samples were tested that were previously exposed to deicing solutions the water absorption could be influenced. The electrical conductivity work was performed as a potential method to develop the understanding of rapid test techniques for quality control. The research used a modified parallel law to relate the electrical conductivity to the pore volume, pore solution conductivity and the tortuosity through the pore network. The influence water addition was able to be determined using electrical conductivity. In addition, the pore solution was observed to be approximately linearly related to the degree of hydration. It is critical that a correction be applied to samples tested at different temperatures. An activation energy of conduction was observed that was approximately 10 kiloliters per mole irrespective of water to cement ratio. In addition to the measurement of transport properties, the relative humidity was assessed for concrete exposed to different exposure conditions. The samples considered in this investigation included a sample stored at 50% relative humidity, covered concrete, a concrete with an exposed vertical surface, a concrete on a drainable base, a concrete on a non-drainable base, and concrete that was submerged. The samples showed that for practical field samples the relative humidity in the concrete was always above 80% for the samples tested. The samples that were exposed to precipitation events demonstrated higher relative humidities.

The role of deicing salts on the non-linear moisture diffusion coefficient of cementitious materials during drying

The drying of cementitious materials is of interest in volume change (i.e., shrinkage) research. However, the movement of water due to drying and wetting also plays a significant role in many durability related problems (e.g., corrosion, alkali silica reactivity, freezing and thawing). Many factors can influence the drying and wetting process in concrete including: pore structure, environmental conditions, and liquid properties. This paper describes the influence of the liquid properties on the drying process. Specifically, this work examines the non-linear moisture diffusion coefficient that is used in a differential equation that describes drying. This paper describes how the non-linear moisture diffusion coefficient is influenced by the presence of deicing salts solutions. The relationship between the equilibrium relative humidity and the solution properties is also discussed in this paper. A higher degree of saturation was observed for the samples containing deicing salt solutions, as compared to the plain samples at any given humidity. The presence of deicing salt causes a shift of the non-linear moisture diffusion coefficient as a function of relative humidity. The non-linear moisture diffusion coefficient curves have near zero rates of drying at low relative humidity with a rapid increase in drying rate as the relative humidity is increased (especially near the equilibrium relative humidity) followed by diffusion coefficient of 0 between RHeq and 100% RH.

Neutron Radiography Measurement of Salt Solution Absorption in Mortar

Some concrete pavements in the US have recently exhibited premature joint deterioration. It is hypothesized that one component of this damage can be attributed to a reaction that occurs when salt-laden water is absorbed in the concrete and reacts with the matrix. This study examines the absorption of CaCl2 solution in mortar via neutron imaging. Mortar specimens were prepared with water to cement ratios, (w/c), of 0.36, 0.42 and 0.50 by mass and exposed to chloride solutions with concentrations ranging from 0 % to 29.8 % by mass. Depth of fluid penetration and moisture content along the specimen length were determined for 96 h after exposure. At high salt concentration (29.8 %), the sorption rate decreased by over 80 % in all samples. Along with changes in surface tension and viscosity, CaCl2 reacts with the cement paste to produce products (Friedels salt, Kuzels salt, or calcium oxychloride) that block pores and reduce absorption.

Early Detection of Joint Distress in Portland Cement Concrete Pavements

Indiana Department of Transportation (INDOT) (as well as several surrounding states) have observed that certain concrete pavements may show a susceptibility to joint deterioration. Unfortunately, by the time that this joint deterioration is observed it is often too late and costly partial depth repairs are needed. The deterioration is generally occurring in the joint behind the backer rod and joint sealant; as such, it is difficult to detect even if one is standing directly above the joint. This project investigated the use of electrical resistivity and ground penetrating radar as two techniques to detect premature joint deterioration. The thought process was that if the joint deterioration is determined at an early stage, low cost corrective actions can be taken to extend the life of the concrete. The electrical response was measured for mortars subjected to a temperature cycle from 23 °C to ‐35 °C, with varying degrees of saturation, and varying salt concentrations. The resistivity increased as the degree of saturation was reduced due to the reduction in the volume of the conductive medium and increase in tortuosity. Changes in resistivity were detected when cracking occurred in the sample. The magnitude of these changes was similar to that detected using changes in the ultrasonic wave speed. Ground penetrating radar (GPR) was used effectively to detect fluid accumulation in the saw‐cut joint behind the joint sealant. The typical GPR waveforms are however difficult and time consuming to interpret. A signal processing approach called complexity-invariance distance, referred to as the CID, was used to obtain a single number that reflects the potential for fluid in the joint. Scalar waveform features and the computed CID can be used to estimate which joints may contain fluid thereby providing insights into which joint sealant sections may need to be repaired or when a sufficient number of joints may contain fluid suggesting a larger joint maintenance effort be performed to seal the joints or the concrete.

Surface and Uniaxial Electrical Measurements on Layered Cementitious Composites having Cylindrical and Prismatic Geometries

Electrical measurements are becoming a common method to assess the transport properties of concrete. For a saturated homogenous system, the surface resistance and the uniaxial resistance measurements provide equivalent measures of resistivity once geometry is appropriately taken into account. However, cementitious systems are not always homogenous. This article compares bulk and surface resistance measurements in cementitious materials intentionally composed of layered materials (i.e., layers with different resistivities). For this study, layered systems were composed of paste and mortar layers, representing the heterogeneity that can exist in the surface layers of field applications as a result of differences in moisture content, segregation, ionic ingress, carbonation, finishing operations, or ionic leaching. The objective of this article is to illustrate that these electrical measures can differ in layered systems (with sharp layer boundaries) and to demonstrate the impact of the surface layer properties on the estimation for the underlying material properties, for both cylindrical and prismatic specimens. Accounting for the effects of a surface layer requires a separate correction in addition to the overall specimen geometry corrections.

Assessing a concrete's resistance to chloride ion ingress using the formation factor

Abstract Concrete in service environments is often exposed to chloride ions which can accelerate the degradation process by causing corrosion of the reinforcing steel. Resistivity measurements have been gaining interest as a method to assess concrete quality. This chapter will discuss why the formation factor, a quantifiable material property that describes the pore network and its connectivity, is the preferred method to assess concrete quality. The chapter discusses how the formation factor can be easily calculated by using an electrical measurements and knowledge of the pore solution properties and how the measured formation factor can be used as an estimation of service life.

Characterizing the Pore Structure of Carbonated Natural Wollastonite

This paper focuses on examining the pore structure of a cementitious paste made with a calcium silicate (wollastonite) that reacts with carbon dioxide and water to form a hardened solid. The pore structure of the hardened solid has been characterized using vapor sorption and desorption, low-temperature differential scanning calorimetry (LT-DSC), and scanning electron microscopy (SEM). The total porosity was also measured using mass measurement in oven-dry and vacuum-saturated conditions. Evidence exists that support the hypothesis that the solid has two main pore sizes: large macropores (>10 nm) appear to form between the initial calcium silicate particles and small micropores (<10 nm) were found in the reacted silica gel. The bimodal nature of the pore structure was evident from the desorption and LT-DSC responses. The extent of reaction was also investigated and was found to be the result of the function of the raw material particle size: only particles with radius <10 μm were found to have entirely reacted even in highly reacted systems. Moreover, the degree of reaction influenced the uniformity of reaction across the sample. Only the highly reacted system showed a uniform microstructure with continuous reaction products path and low porosity.